Optical amplifier and optical wavelength division multiplexing communication system

ABSTRACT

The present invention provides a wavelength division multiplex light communication system using an optical amplifier which can amplifying and translating a multiplex light signal obtained by wavelength division multiplexing a plurality of channel light signals and can perform its control such that when the number of channels n is the maximum number of channels n max , the total light output power P t  is maximum, P max  and such that when the number of channels n for light signals is smaller than n max , the total light output power P t  obtained by amplifying said multiplex light signal is substantially 
     
       
           Pt&gt;P   max   ×n/n   max .

BACKGROUND OF THE INVENTION

The present invention relates to an optical amplifier and wavelengthdivision multiplex light communication system.

FIG. 9 illustrates a wavelength division multiplex light communicationsystem according to the prior art while FIG. 10 is a graph schematicallyshowing the relationship between the number of channels in an opticalamplifier used in the wavelength division multiplex light communicationsystem of the prior art and the total light output power.

As shown in FIG. 9, the wavelength division multiplex lightcommunication system of the prior art comprises a plurality (eight inthis figure) of light transmitters 1 for transmitting light signals, amultiplexer 2 for wavelength division multiplexing a plurality ofchannel light signals transmitted from the light transmitters 1, aplurality of optical amplifiers 3 connected in series with one anotherfor amplifying and translating the multiplex light signals wave divisionmultiplexed by the multiplexer 2, a splitter 4 for wavelength separatingthe amplified light signals from the optical amplifiers 3 for eachchannel and a plurality of light receivers 5 each for receiving eachlight signal wavelength separated by the splitter 4.

In general, the optical amplifier used in the wavelength divisionmultiplex light communication system is adapted to increase or decreasethe total light output power depending on the number of channels.

SUMMARY OF THE INVENTION

The present invention provides an optical amplifier characterized bythat the control is performed such that the total light output power Ptbecomes equal to the maximum total light output power P_(max) when thenumber of channels (n) is maximum (n_(max)) and also such that the totallight output power P_(t) obtained by amplifying multiplex light signalssubstantially becomes

 P _(t) >P _(max) ×n/n _(max)

when the number of light signal channels n is smaller than n_(max).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wavelength division multiplex light communicationsystem according to one embodiment of the present invention.

FIG. 2(a) is a graph illustrating the relationship between the number ofchannels and the total light output power in the first optical amplifieraccording to the embodiment of the present invention while FIG. 2(b) isa diagram illustrating the I-L (incoming current-output level)characteristics of the pump light source.

FIG. 3 is a graph schematically illustrating the relationship betweenthe number of channels and total light output power in the secondoptical amplifier according to the embodiment of the present invention.

FIG. 4 is a graph illustrating the relationship between the number ofchannels and the total light output power in an optical amplifier usedwith a maximum eight-channel wavelength division multiplex lightcommunication system, in comparison with this embodiment with the priorart.

FIG. 5 is a graph illustrating the relationship between the number ofchannels and the total light output power in another optical amplifierused with a maximum eight-channel wavelength division multiplex lightcommunication system, in comparison with this embodiment with the priorart.

FIG. 6 is a graph illustrating the relationship between the number ofchannels and the total light output power in an optical amplifier usedwith a maximum 16-channel wavelength division multiplex lightcommunication system, in comparison with this embodiment with the priorart.

FIG. 7 shows graphs illustrating span loss-Q value (Quality Factor)characteristics of optical amplifiers to be controlled to the totallight output power shown in FIG. 4: FIG. 7(A) being a graph illustratingthe span loss-Q value characteristics in the prior art optical amplifierfor two-wave (two-channel) and eight-wave (eight-channel) transmissionsand FIG. 7(B) being a graph illustrating the span loss-Q valuecharacteristics in the optical amplifier of this embodiment for two-waveand eight-wave transmissions.

FIG. 8 shows graphs illustrating span loss-BER (Bit Error Rate)characteristics of optical amplifiers to be controlled to the totallight output power shown in FIG. 4: FIG. 8(A) being a graph illustratingspan loss-BER characteristics in the prior art optical amplifier fortwo-wave and eight-wave transmissions and FIG. 8(B) being a graphillustrating span loss-BER characteristics in the optical amplifier ofthis embodiment for two-wave and eight-wave transmissions.

FIG. 9 shows a wavelength division multiplex light communication systemaccording to the prior art.

FIG. 10 is a graph schematically illustrating the relationship betweenthe-number of channels and the total light output power in an opticalamplifier used with the wavelength division multiplex lightcommunication system of the prior art.

DETAILED DESCRIPTION

The embodiments of the present invention will now be described incomparison with the prior art by way of example with reference to thedrawings.

In the prior art, the optical amplifier normally increased the totallight output power Pt proportional to the number of channels (n), asshown in FIG. 10. This is mainly for two following reasons.

(1) In order to ensure a good signal-to-noise (S/N) ratio, it isrequired that an optical amplifier in any subsequent stage receives asufficiently high light output power for each channel. Therefore, thelight output power for each channel must exceed a predeterminedacceptable minimum level.

(2) The light output power for each channel should not exceed apredetermined acceptable maximum level so that any non-linear effectsuch as stimulated brillouin scattering (SBS) will not occur in theoptical fiber.

However, the optical amplifier of the prior art cannot necessarilysatisfy the above requirement (1) if the total light output power causesto be increased proportional to the number of channels. The reason willbe described in detail.

The total light output power (P_(t)) in the optical amplifier isrepresented by the sum of light output powers (P_(j): j being channelnumber) for every channel plus the light output power (P_(ASE)) of aspontaneously emitted light (ASE light) occurred in the opticalamplifier:

Pt=ΣPj+P _(ASE).

If the gain is the same throughout the optical amplifiers, the magnitudeof P_(ASE) is substantially invariable. Consequently, the light outputpower for each channel is smaller as the number of channel is smaller.The contribution of P_(ASE) relative to Pt is correspondingly increased.For such a reason, the wavelength division multiplex light communicationsystem comprising a plurality of optical amplifiers connected in serieswith one another will provide a considerably large contribution ofP_(ASE) in comparison with P_(j). Since the fraction of light outputpower taken by P_(ASE) becomes larger as the number of channels issmaller as long as the total light output power P_(t) is proportional tothe number of channels (n), the light output power P_(j) for eachchannel will become smaller. As a result, P_(j)>P_(min) (acceptableminimum value) may not be satisfied when the number of channels issmaller, rather than when the number of channels is larger.

Rather than the larger number of channels, the smaller number ofchannels did not provide a good transmission since the system was moregreatly influenced by the ASE light with the total light output powerbeing more easily variable to degrade S/N ratio and others.

Moreover, the optical amplifier of the prior art controls the totallight output power depending on increase or decrease of the number ofchannels. If the number of channels is increasing, however, the controlof the optical amplifier cannot follow such a condition. Thistemporarily makes the gain in the existing channels insufficient, sothat a good S/N ratio cannot be ensured. The reason will be described indetail.

For example, if the number of channels increases from n to n+1 and whenthe optical amplifier recognizes the increase of the number of channelsfrom information from any monitoring system, the optical amplifierincreases its total light output power from P_(n) to P_(n+1).

Normally, there is a delay time ranging between several msec and severalten msec until the total light output power increases after the numberof channels has increased. During this, the output of the opticalamplifier will remain at the value of P_(n) regardless of the number ofchannels becoming n+1.

This may occur such a situation that the light output power for eachchannel does not exceed the predetermined acceptable minimum level andmay thus occur the temporal degradation of S/N ratio such as bit errorthe like. Such a situation is most serious occurs as the number ofchannels increases from 1 to 2. In this case, there may temporarilyoccur such a situation that the light output power becomes half or P₀ inthe transient condition in which the number of channels is increasing,regardless of the total light output power of 2×P₀ in the stationarytwo-channel state.

FIG. 2(a) is a graph illustrating the relationship between the number ofchannels and the total light output power in the first optical amplifieraccording to the embodiment of the present invention. In this figure,solid lines represent the case of the first optical amplifier which isone embodiment of the present invention while alternate long and shortdash lines represent the case of an optical amplifier according to theprior art.

As shown in FIG. 2(a), the first optical amplifier performs its controlsuch that when the number of channels n is maximum or eight, the totallight output power P_(t) becomes equal to the maximum total light outputpower P_(max). If the number of channels is smaller than the firstnumber of channels of 5 or ranges between 1 and 4, the first opticalamplifier performs its control such that the total light output powerP_(t) is substantially

P _(t) >P _(max) ×n/8,

and if the number of channels is equal to or larger than 5 or the firstnumber of channels, the first optical amplifier performs its controlsuch that the total light output power P_(t) is substantially

P _(t) =P _(max) ×n/8,

as in the prior art optical amplifier.

The first optical amplifier further performs its control such that whenthe number of channels is between 1 and 2 (the second number ofchannels), the total light output power P_(t) is maintainedsubstantially at the value of the total light output power P_(t2) on thesecond number of channels or 2.

For example, if an optical amplifier is a so-called optical fiberamplifier in which an optical fiber added with a rare earth element suchas Er or the like is pumped by injecting an pump light thereinto from anpump light source so that the optical fiber can pass and amplify signallights, the relationship between the injecting current of the pump lightsource and the light output level can be described as follows.

FIG. 2(b) is an I-L characteristic diagram in which the injectingcurrent I for controlling said pump light source is on horizontal axiswhile the light output level L from said pump light source is onvertical axis. In this figure, a spontaneous emission light is emittedfrom the pump light source when the injection current I is between 0 andthe laser oscillation threshold current Is. Since the output level atthis time is relatively low and thus no laser beam of the necessarywavelength will be obtained, the optical amplifier cannot be controlled.As the injection current I exceeds Is, the laser oscillation begins suchthat a laser beam having a predetermined wavelength will be outputtedfrom the pump light source with the output level L proportional to theinjection current I. Thus, the total light output power P_(t) of theoptical amplifier can be increased or decreased proportional to theamount of injection current I at the pump light source.

Numerals on the top of FIG. 2(b) represent the number of channelscorresponding to the controls in both the prior art and the embodimentof the present invention. Each of these numerals corresponds to thevalue of the injection current I indicated just therebelow.

In other words, the control of the prior art shown in FIG. 2(a)increases both the injection current I and output level L as the numberof channels increases from one channel to eight channels, as shown inFIG. 2(b). In the control of the present invention shown in FIG. 2(a),both the injection current I and output level L are substantially thesame as those of three channels in the prior art when the number ofchannels is 1 or 2; as those of four channels in the prior art when thenumber of channels is three; as those of five channels in the prior artwhen the number of channels is 4 or 5; and as those of six to eightchannels when the number of channels is between six and eight.

The optical amplifier of the prior art shown in FIG. 2(a) controls thetotal light output power P_(t) proportional to the number of channels n.In order to utilize the linear sections in the I-L characteristics aseffectively as possible, the injection current I corresponding to one ortwo channels in the prior art was controlled to be very close to thelaser oscillation threshold current Is. With one or two channels,therefore, the injection current I becomes too small. Thus, the accuratecontrol cannot be performed, with the amplification in the opticalamplifier being destabilized. If the number of channels is changed fromtwo to one, for example, the injection current I may temporarily becomelower than the laser oscillation threshold current Is. This may notcontrol the optical amplifier.

On the contrary, this embodiment of the present invention performs itscontrol such that when the number of channels is one or two, the totallight output power P_(t) is maintained constant at the total lightoutput power corresponding to two channels (which is substantially thesame as that of three channels in the prior art).

Even though the number of channels is changed between one and two,therefore, it is not necessary to change the injection current I even ifit is finely adjusted. Thus, the injection current I can also bemaintained as a relatively high level so that the control of the opticalamplifier can be stabilized.

If the number of channels n is smaller than eight, the first opticalamplifier controls the total light output power such that it is higherthan the total light output power proportional to the number ofchannels. Thus, the contribution of ASE light can be compensated toensure a good S/N ratio. Even though the gain control of the opticalamplifier cannot follow increase or decrease of the number of channels,the gain of the existing channel can be prevented from being madeinsufficient due to influence of the increase or decrease of channel,since the gain of the existing channel is controlled to be larger thanthe inherently necessary level.

The first optical amplifier can improve the S/N ratio and reduce theelectric power consumed by the optical amplifier on the side of lesschannels (one to four channels) and will not perform the control withany unnecessarily high gain on the side of more channels (five to eightchannels). Thus, the electric power consumed by the optical amplifiercan be reduced. Therefore, the control of the optical amplifier can beoptimized from the viewpoint of S/N ratio and consumed power.

Moreover, the first optical amplifier can maintain the total lightoutput power P_(t) substantially constant at the total light outputpower P_(t2) corresponding to the second number of channels equal to twowhen the number of channels n is changed from one to two in the secondnumber of channels. Thus, the light output from the pump light sourcecan be stabilized to make the operation of the optical amplifier stableand to improve the reliability.

FIG. 3 is a graph schematically illustrating the relationship betweenthe number of channels and total light output power in the secondoptical amplifier according to the embodiment of the present invention.In this figure, solid lines represent the case of the second opticalamplifier according to this embodiment of the present invention whilealternate long and short dash lines represent the case of the prior artoptical amplifier.

As shown in FIG. 3, the second optical amplifier controls the totallight output power P_(t) such that it is substantially

P _(t) =P 0+(P _(max) −P 0)×n/8

(wherein P0 is a constant value).

If the number of channels n is smaller than eight, the second opticalamplifier controls the total light output power such that it becomeshigher than the total light output power proportional to the number ofchannels. Thus, the contribution of ASE light can be compensated toensure a good S/N ratio. Even though the gain control of the opticalamplifier cannot follow increase or decrease in the number of channels,the gain of the existing channel can be prevented from being madeinsufficient due to influence of the increase or decrease of channel,since the gain of the existing channel is controlled to be larger thanthe inherently necessary level.

The second optical amplifier can more easily be controlled than thefirst optical amplifier since the total light output power P_(t) can becalculated by the same formula without dependent on the number ofchannels.

FIG. 1 shows a wavelength division multiplex light communication systemaccording to one embodiment of the present invention. In this figure,parts similar to those of FIG. 9 have similar reference numerals. As canbe seen from FIG. 1, this wavelength division multiplex lightcommunication system is characterized by that it comprises theaforementioned optical amplifier 3 a or optical amplifiers 3 a and 3 b.

For example, the optical amplifiers 3 a other than the final stageoptical amplifier 3 b on the light receiver side perform their controlsusing said first or second optical amplifier. The final stage opticalamplifier 3 b performs its control such that it provides

 P _(t) =P _(max) ×n/n _(max)

for all the numbers of channels, as in the prior art optical amplifier.In other words, the final stage optical amplifier 3 b performs itscontrol with the total light output power so determined that lightsignals of the respective wavelengths based on the multiplex lightsignals outputted from the final stage optical amplifier 3 b become alevel adapted to each of the light receivers 5 for receiving the lightsignals. In this case, even though each of the light receivers 5 isconventionally available, it can receive light signals without anyproblem by causing only the final stage optical amplifier 3 b to controlthe gain considering the limit of reception at the light receivers 5,independently of the control of the other optical amplifiers 3 a.

In the other form, all the optical amplifiers 3 a, 3 b may perform thecontrol using said first or second optical amplifier. In such a case,for example, a light attenuator (ATT) 6 may be provided in front of eachof the light receivers 5. Such a light attenuator 6 can attenuate andregulate light signals from the respective light receiver. Even if eachof the light receivers 5 is conventionally available, therefore, it canreceive light signals without any problem.

If the maximum number of channels is changed, moreover, each of theoptical amplifiers 3 a and 3 b can be controlled to reset the totallight output power for each number of channels.

FIGS. 4-6 are graphs illustrating the relationship between the number ofchannels and the total light output power (or calculated value) in theoptical amplifier used with the maximum eight-channel wavelengthdivision multiplex light communication system of the present invention,in comparison with the present invention with the prior art. Conditionsof light transmission path include:

(1) Type of optical fiber: single mode;

(2) Bit rate per channel: about 2.4 Gbps; and

(3) One span: 33 dB (which corresponds to about 120 km).

FIG. 4 shows the case of an optical amplifier used in the wavelengthdivision multiplex light communication system having eight channels inmaximum. As shown in FIG. 4, this optical amplifier performs its controlsuch that the total light output power for 5-8 channels is substantiallythe same as that of the conventional optical amplifier and that thetotal light output power for 1-4 channels is larger than that of theconventional optical amplifier.

FIG. 5 shows the case of another optical amplifier used in thewavelength division multiplex light communication system having eightchannels in maximum. As shown in FIG. 5, this optical amplifier performsits control such that the total light output power for 2-8 channels issubstantially the same as that of the conventional optical amplifier,and such that the total light output power for one channel is largerthan that of the conventional optical amplifier and substantially thesame as that of the total light output power for two channels.

FIG. 6 shows the case of an optical amplifier used in the wavelengthdivision multiplex light communication system having sixteen channels inmaximum. As shown in FIG. 6, this optical amplifier performs its controlsuch that the total light output power for 9-16 channels issubstantially the same as that of the conventional optical amplifier,and such that the total light output power for 1-8 channels is largerthan that of the conventional optical amplifier and substantially thesame as that of the total light output power for one and two channels.

FIG. 7 shows graphs illustrating the span loss-Q value (Quality Factor)characteristics of an optical amplifier for controlling the total lightoutput power shown in FIG. 4. FIG. 7(A) is a graph illustrating the spanloss-Q value characteristics in the prior art optical amplifier fortwo-wave (two-channel) and eight-wave (eight-channel) transmissions.FIG. 7(B) is a graph illustrating the span loss-Q value characteristicsin the optical amplifier of this embodiment for two-wave and eight-wavetransmissions. The measurements were carried out under such a conditionin which the number of spans is five; the measuring channel is λ3; andthe Q value of the light transmitter output of λ3 is 27.74. In FIG. 7,the horizontal axis represents span loss while the vertical axisrepresents Q value after five spans have been transmitted.

In comparison between FIGS. 7(A) and 7(B), it is found that, foreight-wave transmission, the embodiment of the present invention hassubstantially the same characteristics as in the prior art. However, fortwo-wave transmission, the embodiment of the present invention has anincreased Q value to provide a good transmission quality.

FIG. 8 shows graphs illustrating span loss-BER (Bit Error Rate)characteristics of optical amplifiers to be controlled to the totallight output power shown in FIG. 4. FIG. 8(A) is a graph illustratingspan loss-BER characteristics in the prior art optical amplifier fortwo-wave and eight-wave transmissions and FIG. 8(B) is a graphillustrating span loss-BER characteristics in the optical amplifier ofthis embodiment for two-wave and eight-wave transmissions. Themeasurements were carried out under such a condition that the number ofspans is five; the measurement channel is λ3; and the input power of thelight receiver is −15.8 dBm. In FIG. 8, the horizontal axis representspan loss while the vertical axis represents BER.

In comparison between FIGS. 8(A) and 8(B), it is found that, foreight-wave transmission, the embodiment of the present invention hassubstantially the same characteristics as in the prior art. However, fortwo-wave transmission, the embodiment of the present invention has areduced BER to provide a good transmission quality.

The present invention is not limited to the aforementioned embodiments,but may be carried out in any of various other forms without departingfrom the scope of the invention as defined in the accompanying claims.

For example, if the number of channels n is smaller than n_(max), anyoptical amplifier other than the aforementioned optical amplifiers maybe used when the control is performed such that the total light outputpower is higher than the total light output power proportional to thenumber of channels in the conventional optical amplifier.

The optical amplifier of the present invention can compensate thecontribution of ASE light and ensure a good S/N ratio since if thenumber of channels n is smaller than n_(max), the control is carried outsuch that the total light output power is higher than the total lightoutput power proportional to the number of channels. Even though thegain control of the optical amplifier cannot follow the increase ordecrease of channel, the present invention can prevent the gain of theexisting channel from being made insufficient due to the influence ofchannel increase or decrease.

According to the present invention, the S/N ratio can be improved on theside of smaller number of channels to reduce the electric power consumedby the optical amplifier. Since the gain control will not unnecessarilybe performed on the side of larger number of channels, the electricpower consumed by the optical amplifier can similarly be reduced. In theviewpoint of S/N ratio and consumed power, the control of the opticalamplifier can be optimized.

According to the present invention, when the number of channels n isbetween one and the second number of channels n2 (n2<n_(max)), the totallight output power P_(t) is substantially maintained constant at thevalue of the total light output power P_(t2) for the second number ofchannels n2. Thus, the light output from the pump light source will bestabilized to make the operation of the optical amplifier stable and toimprove the reliability.

The wavelength division multiplex light communication system of thepresent invention includes the above optical amplifier According to thepresent invention, even though each of the light receivers isconventionally available, it can receive light signals without anyproblem since only the final stage optical amplifier is caused tocontrol the gain considering the limit of reception at the lightreceivers, independently of the control of the other optical amplifiers.

If all the optical amplifiers except the final stage optical amplifierare in the form of the aforementioned optical amplifier, the wavelengthdivision multiplex light communication system of the present inventioncan perform the maximum function.

What is claimed is:
 1. An optical amplifier, for amplifying andtranslating a multiplex light signal obtained by wavelength divisionmultiplexing a plurality of channel light signals with a total lightoutput power corresponding to the number of light signal channels,wherein the control is performed such that the total light output powerPt becomes equal to the maximum total light output power P_(max) whenthe number of channels (n) is maximum (n_(max)) and also such that thetotal light output power P_(t) obtained by amplifying multiplex lightsignals substantially becomes P _(t) >P _(max) ×n/n _(max) when thenumber of light signal channels n is smaller than n_(max).
 2. Theoptical amplifier of claim 1, wherein the control is performed such thatif the number of channels n is smaller than the first number of channelsn1 (n1<n_(max)), the total light output power P_(t) becomessubstantially P _(t) >P _(max) ×n/n _(max), and such that if the numberof channels n is equal to or larger than the first number of channelsn1, the total light output power P_(t) becomes substantially P _(t) =P_(max) ×n/n _(max).
 3. The optical amplifier of claim 1 or 2, whereinthe control is performed such that when the number of channels n isbetween 1 and the second number of channels n2 (n²<n_(max)), the totallight output power Pt substantially becomes constant at the total lightoutput power P_(t2) obtained by the second number of channels n2.
 4. Awavelength division multiplex light communication system comprising: aplurality of light transmitters for transmitting light signals; amultiplexer for wavelength division multiplexing a plurality of channellight signals transmitted from said light transmitters; a plurality ofoptical amplifiers connected in series with one another for amplifyingand translating the multiplex light signals wavelength divisionmultiplexed by said multiplexer, wherein at least any of said opticalamplifiers being any of said optical amplifiers of claim 1 to 3; asplitter for wavelength separating the light signals amplified by saidoptical amplifiers for each channel; and a plurality of light receiversfor receiving the light signals wavelength separated by said splitter.5. The wavelength division multiplex light communication system of claim4, wherein the final stage optical amplifier adjacent to said lightreceivers may perform its control with a total light output powerdetermined such that the light signal of the respective wavelengthoutputted from said final stage optical amplifier and wavelengthseparated by said splitter becomes a level fitting in the light receiverreceived said light signal.